This invention relates to a magnetooptical recording medium that is capable of direct overwriting by light intensity modulation without using a separate source for generating an external magnetic field.
A prior art magnetooptical recording medium capable of light-modulated overwriting is described in Journal of Applied Physics, Vol. 67, No. 9, pp. 4415-4416 and its basic structure is shown diagrammatically in FIG. 1 together with the magnetic field generation source and light beam for recording and reproduction purposes; shown by 1 is a light beam that issues from a light source such as a laser and that is focused by a lens; 2 is the source of generating a magnetic field Hb; 3 is a transparent glass or plastic substrate; 4-7 are four magnetic layers that are bound together by exchange force. The first layer 4 stores binary-level information 0 or 1 depending upon whether magnetization is directed upward or downward. In a reproduction mode, a laser beam is applied and the resulting magnetooptical Kerr effect causes the plane of polarization to rotate, whereupon the direction of magnetization is read out as binary-level information. The second to fourth layers 5-7 are necessary for achieving light-modulated overwriting. In particular, the fourth layer 7, after it is formed, is magnetized over the entire surface in a uniform direction, say, upward. Since the fourth layer 7 has an extremely high Curie point and a large coercive force, it will not experience reversal of magnetization in either a recording or reproduction mode and instead it will maintain the upward direction of magnetization almost indefinitely. The specific actions of the second and third layers 5 and 6 will be described later in this specification. The Curie points of the four layers generally have the following relationship (with the Curie point of the i-th layer being denoted by Tci): EQU Tc3&lt;Tc1&lt;Tc2&lt;Tc4.
The mechanism of direct overwriting is described below with reference to FIG. 2. FIG. 2A shows diagrammatically how the magnetization of each layer changes in a recording mode, and FIG. 2B is a graph showing the intensity profile of a laser beam during recording and reproduction. As shown in FIG. 2B, the laser beam can take on three intensity levels, P.sub.R, P.sub.L and P.sub.H (R, read; L, low; H, high). P.sub.R which represents the intensity level in a reproduction mode is so small that it will not cause any change in the state of magnetization of the medium. On the other hand, P.sub.L and P.sub.H which represent the intensity levels of the laser beam applied in a recording mode are large enough to change the state of magnetization of the medium. These two intensity levels satisfy the relation P.sub.H &gt;P.sub.L. When "0" of binary-level information is to be recorded, the laser beam is applied at the intensity level P.sub.L and, if "1" is to be recorded, the laser beam is applied at the intensity P.sub.H. When the medium is illuminated with P.sub.L, its temperature rises up to T.sub.L and if it is illuminated with P.sub.H, its temperature rises up to T.sub.H (T.sub.H &gt;T.sub.L).
The area of the medium thus heated by illumination with a laser beam spot starts to cool when the illumination ends. FIG. 2A shows schematically how the magnetization of each layer is reversed in this cooling process; the upper column shows the state of magnetization reversal during cooling after illumination with P.sub.L and the lower column refers to cooling after illumination with P.sub.H.
Before describing in detail the changes of magnetization that are shown in FIG. 2A, let us make a brief review of the basic properties of rare earth/transition metal (hereunder designated RE-TM) systems which are commonly used as magnetooptical materials. RE-TM systems are generally referred to as ferrimagnetic materials and RE is bound to TM in such a way that their magnetizations cancel each other (i.e., in an antiparallel fashion). Hence, the overall magnetization is oriented in the direction of whichever the greater of the magnetizations of RE and TM and its strength is determined by the difference between the two magnetizations. If the magnetization of RE is the stronger, the magnetooptical material of interest is said to be "RE dominant" and, in the opposite case, it is said to be "TM dominant". A composition where the overall magnetization is zero is given a special name, "compensated composition". As the temperature increases, the magnetizations of both RE and TM decrease; however, since the magnetization of RE decreases more rapidly than the magnetization of TM, there is a tendency for the overall magnetization to shift from "RE dominant" to "TM dominant" state as the temperature increases.
In the next place, we describe the properties of a multilayered film in which the individual layers are bound exchange force. "Exchange force" refers to the force that works between adjacent magnetic layers in such a way that the direction of magnetization of TM in one layer is parallel to that of magnetization of TM in the adjacent layer. Take, for example, the case where the magnetization of TM in the fourth layer is directed downward; then, exchange force acts in such a way TM in the third layer will be magnetized in the same direction (downward). Needless to say, the magnetization of TM is antiparallel to that of RE in each layer, one may as well say that exchange force acts in such a way as to create parallelism in the magnetization of RE in two adjacent layers.
Going back to FIG. 2A, we now describe the mechanism of overwriting. The direction of arrow in each of the layers shown in FIG. 2A refers to the direction of magnetization of TM in each layer.
At room temperature, the magnetization of TM is directed upward in each of the four layers 4-7, except that magentization in the first layer 4 is directed either upward or downward depending upon the binary-level information to be recorded (State 3 or 7).
When the information to be recorded is "0" (i.e., if the magnetization of TM in the first layer is to be directed upward), a laser beam having intensity P.sub.L is applied and the temperature of the medium will rise to T.sub.L (.gtoreq.Tc1). Since T.sub.L is higher than the Curie points of both the first layer 4 and the third layer 6, their magnetization disappears (State 1). If the medium is cooled to a temperature below Tc1, the magnetization of TM in the first layer 4 is so oriented by exchange force that it is directed downward, i.e., parallel to the magnetization of TM in the second layer (State 2). If cooling proceeds until the temperature is close to room temperature, the first layer 4 becomes stable and the recording of "0" is completed (State 3).
When the information to be recorded is "1" (i.e., if the magnetization of TM in the first layer is to be directed downward), a laser beam having intensity P.sub.H is applied and the temperature of the medium will rise to T.sub.H (.gtoreq.Tc2). Since T.sub.H is higher than the Curie points of the first, second and third layers 4-6, their magnetization disappears (State 4). If the medium is cooled to a temperature below Tc2, it is magnetized in a downward direction by Hb, or magnetization applied externally in a downward direction. At the WRITE temperature under consideration, the second layer 5 is TM dominant, so the magnetization of TM is directed downward as is the overall magnetization (State 5). In this state, the magnetization of the third layer 6 having the lowest Curie point has of course disappeared and this blocks the exchange force acting from the fourth layer 7 to the second layer 5. If the third layer 6 were absent, the exchange force would act from the fourth layer 7 to the second layer 5. This is the force that will render the magnetization of TM in the second layer 5 to be directed upward in such a way as to impede the action of the bias magnetic field Hb. Therefore, it is due to the presence of the third layer 6 that binary-level information can be smoothly written into the second layer 5 in State 5 even in a small bias magnetic field Hb.
If cooling proceeds further than State 5 and the temperature becomes lower than Tc1, the Curie point of the first layer 4, the exchange force acts to orient the magnetization of TM in the first layer 4 in such a way that it is directed downward in alignment with the magnetization of TM in the second layer 5 (State 6). If the temperature further decreases to become lower than Tc3, the Curie point of the third layer 5, the exchange force starts to act from the fourth layer 7 to the third layer 6, orienting the magnetization of TM in the third layer 6 to be directed upward. The exchange force also starts to act from the third layer 6 to the second layer 5, reverting the magnetization of TM in the second layer 4 to be directed upward (State 7). In this state, the exchange force also acts from the second layer 5 to the first layer 4; however, the first layer 4 becomes very stable as room temperature is approached, so that it overcomes the exchange force to retain the present direction of magnetization, whereby the recording of "1" is completed.
As described above, the recording of "0" or "1" is accomplished by modulating the intensity of a laser beam to P.sub.L or P.sub.H. In other words, direct overwriting is performed by light modulation.
However, the prior art magnetooptical recording media have the following various problems.
(i) First, they require a separate source of generating a bias magnetic field in a recording mode and this has increased the complexity of the equipment.
The present invention has been accomplished in order to solve this problem and has as an object providing a magnetooptical recording medium that is capable of light-modulated direct overwriting without requiring any separate source of generating a magnetic field.
(ii) A second problem with the prior art magnetooptical recording media is that the exchange force acting between the first and second magnetic layers is strong at a temperature where the magnetization of the first layer is aligned to the direction of magnetization of sublattices of transition metals in the second layer whereas it is necessary to reduce the exchange force acting between the first and second magnetic layers in the process of initialization at a temperature near room temperature and because of this small latitude in adjusting the thickness of the first and second magnetic layers, it has been difficult to achieve consistent production of reliable magnetooptical recording media.
The present invention has been accomplished in order to solve this problem and has as an object providing a magnetooptical recording medium that is capable of effectively controlling the exchange force acting between the first and second magnetic layers.
(iii) The prior art magnetooptical recording media use magnetic layers typically made of TbFeCo in consideration of several factors including the exchange force acting between the first and second magnetic layers.
For achieving higher-density recording, extensive studies are being made by designers of magnetooptical recording apparatus to shorten the operating wavelength of the optical head (i.e., semiconductor laser) which is used in both recording and reproduction modes. The prior art magnetooptical recording media have a sufficient Kerr magnetooptical effect for satisfactory reproduction output at or near 800 nm which is currently used to operate the optical head but if the operating wavelength is reduced by half to 400 nm, the angle of rotation will decrease to less than a half of the value that is achievable at 800 nm and no satisfactory reproduction output can be obtained.
The present invention has been accomplished in order to solve this problem and has as an object providing a magnetooptical recording medium that is capable of light-intensity modulated direct overwriting and that yet achieves satisfactory reproduction output even at an operating wavelength of 400 nm which is one half the currently employed value.
(iv) A further problem with the prior art magnetooptical recording media capable of light-modulated direct overwriting is that they require a strong bias magnetic field in order to compensate for the spurious magnetic field originating from the fourth layer. In addition, the magnetization of the fourth layer is reversed during illumination with P.sub.H, thereby making it impossible to perform another overwriting.
The present invention has been accomplished in order to solve this problem and has as an object providing a magnetooptical recording medium that is capable of direct overwriting in a weak bias magnetic field and which yet is characterized by the greater stability of the fourth layer under illumination with P.sub.H.